import math


def isValid(side1, side2, side3):
    a = side1 + side2 > side3
    b = side1 + side3 > side2
    c = side2 + side3 > side1
    if a and b and c:
        return True
    else:
        return False


def area(side1, side2, side3):
    if isValid(side1, side2, side3):
        s = (side1 + side2 + side3) / 2
        __area = math.sqrt(s * (s - side1) * (s - side2) * (s - side3))
        print('The area of the triangle is', __area)
    else:
        print('Input is invalid')


side1,side2,side3 = eval(input())
area(side1,side2,side3)